The present invention relates to an improved method for the polymerization of vinyl chloride monomer in an aqueous medium or, more particularly, to a method for the polymerization of vinyl chloride in an aqueous medium, which method is outstandingly freed from the problem of the polymer scale deposition on the inner walls of the polymerization reactor and surfaces of stirrer and other parts coming into contact with the monomer during the polymerization reaction.
Needless to say, the most widely undertaken method for the polymerization of vinyl chloride or a monomer mixture mainly composed of vinyl chloride is the suspension polymerization of the monomer or monomer mixture in an aqueous medium in the presence of a monomer-soluble polymerization initiator. As is well known, one of the serious problems in the suspension polymerization of vinyl chloride or the like monomer is the deposition of polymer scale on the inner walls of the polymerization reactor and surfaces of the stirrer and other parts coming into contact with the monomer during the polymerization reaction. Once the inner walls of the polymerization reactor have been covered by the polymer scale deposited thereon, the cooling capacity of the polymerization reactor is greatly decreased resulting in the decrease in the productivity and the polymer scale coming off the reactor walls is eventually intermixed with the polymer product to cause degradation of the product quality. In addition, the polymer scale deposited on the surfaces must be removed to prepare for the next run of the polymerization taking a great deal of labor and time while such a cleaning work of the reactor involves a very serious porblem of the workers' health due to the toxicity of the vinyl chloride monomer absorbed in the polymer scale in a considerably large amount.
One of the approaches for preventing polymer scale deposition in the suspension polymerization of vinyl chloride is to provide the reactor walls with a coating of certain compounds insusceptible to the deposition of the polymer scale. Various types of coating materials have been proposed hitherto including polar organic compounds such as amines, quinones, aldehydes and the like, dyes and pigments, reaction products of a polar organic compound or dye with a metal salt, mixtures of an electron donor compound and an electron acceptor compound, inorganic salts or complex compounds, and others.
Besides the above mentioned methods by providing the reactor walls with a coating layer, attempts have been made to decrease the polymer scale deposition by the modification of the recipe of the polymerization mixture introduced into the polymerization reactor. For example, Japanese Patent Kohyo Nos. 57-5000614 and 57-500650 teach admixture of the aqueous medium for the suspension polymerization of vinyl chloride with a dispersing agent which is a water-insoluble crosslinked polymer prepared by the crosslinking copolymerization of acrylic acid and a crosslinkable monomer such as allyl pentaerithritol, allyl saccharose and the like. This method, however, is not quite satisfactory in respect of the rather low effectiveness of the method for the prevention of polymer scale deposition.